Milky Way’s two stellar halos have opposing spins

Washington, Dec.13 (ANI): We call it home, but the Milky Way can still surprise us. It does not have just one halo of stars, but two. According to Daniela Carollo of the Torino Observatory in Italy and her colleagues, the discovery was made while measuring the metal content and motion of 20,000 stars in the Milky Way through the Sloan Digital Sky Survey. Carollo and her team found that the halo can be divided into two distinct regions, rotating in opposite directions, and containing stars of different chemical composition. The team found that the inner halo is flattened and extends out to about 4.6 x 1017 kilometres from the galactic centre, rotating at 20 kilometres per second, in the same sense that the Sun travels round the galactic centre. The outer halo is spherical, stretching out to over 6.0 x 1017 kilometres and spinning in the opposite direction at about 70 kilometres per second. By examining the spectrum of light emitted by the stars, the team also calculated that the inner-halo stars contain three times more heavy atoms than the outer-halo stars, raising questions about when the two halos formed. “The two haloes appear to have been formed at different times by different mechanisms,” believes Carollo. “This result throws out all our current models of galaxy formation,” Carollo reports in her study published in Wednesdays issue of the journal Nature. (ANI)